Two-dimensional (2D) nanomaterials present unique structures and properties for ultrathin electronic and optoelectronic device applications. Recently, rhenium disulfide (ReS2), one of the semiconducting transition metal dichalcogenides (TMDCs), has been shown to have an unusual distorted IT structure with random stacking and weak interlayer coupling. Unlike other TMDCs, such as molybdenum or tungsten disulfide whose bandgaps transition from indirect to direct in the monolayer limit, ReS2 possesses a direct bandgap in the bulk and as a monolayer, which provides advantages in high-gain photodetector applications. While its band structure remains qualitatively unchanged as a function of thickness, ReS2 does show a subtle layer-dependent photoluminescence (PL) blue-shift due to quantum confinement effects.
Previous work has shown that ReS2 nanosheets can be isolated by micromechanical and chemical exfoliation methods. Although micromechanical exfoliation provides high-quality nanosheets, lack of scalability presents serious limitations for real-world applications. Conversely, while chemical exfoliation can produce larger quantities of nanosheets, this process drives a phase transition that necessitates subsequent thermal treatments to attempt to recover the original ReS2 phase.
Liquid-phase exfoliation (LPE) is an alternative, scalable route for isolating 2D nanomaterials without chemical modifications. LPE can yield large quantities of 2D nanomaterials, but ultimately lacks control over structural parameters such as thickness. The structural polydispersity of LPE dispersions is known to introduce numerous problems, particularly in electronic and optoelectronic applications, thus motivating efforts to develop post-exfoliation, solution-based separation methods. Towards this end, isopycnic density gradient ultracentrifugation (iDGU), has been adapted to nanomaterial dispersions including carbon nanotubes and low-density 2D nanomaterials to improve structural and electronic monodispersity. Thus far, this technique has been limited to nanomaterials with buoyant densities in aqueous surfactant solutions lower than the standard density gradient medium iodixanol (ρmax=1.32 g/cm3). With a buoyant density as high as ρmax=1.9 g/cm cesium chloride (CsCl) would appear to be a natural candidate to extend the density range of iDGU. However, with a significantly lower viscosity than iodixanol, density gradients based on CsCl are relatively unstable, which hinders its utility for nanomaterial separations.